By default when we design a User Experience we default to being consistent with some standard (a convention or pattern of UI, a corporate guideline, a previous version, etc.). It's a completely valid approach.

I've seen some people turn this from a default position to an inflexible rule. That is, they will, knowingly or not, sacrifice for consistency even those deviations that, by their own admission, are better for the user.

To What End, Consistency?

The reason we make UI consistent is to Make life better for our users and NOT to be consistent (a tautology).

A Tool, Not a Dogma

Consistency is just one of many tools we have available to create that better experience.

Although I, in general, start from a position of consistency, all things being equal, I'll take an UI inconsistency that makes a better user experience over a consistent UI that makes it worse.

Users can absorb some inconsistency

In some cases, inconsistencies are completely unnoticed by our users.

Example:

Case1 - Drag a file (left click) in Windows Explorer from one hard drive to another. The file is copied.

Case2 - Drag a file (left click) in Windows Explorer from one folder to another folder in the same drive. The file is moved.

Why does it do this? I don't know.

Is it consistent? No.

Does anyone notice? When I've pointed it out to people they were always surprised.

Does the iconsistency ever cause a problem? Not that I have heard. As far as I can tell, Windows Explorer in regard to dragging files "Just Works"